The present application is the U.S. national stage under 35 U.S.C. 371 of International Application No. PCT/DE99/02095 filed Jul. 6, 1999 claiming priority of German Patent Application No. 198 33 025.1 filed Jul. 23, 1998.
The invention relates to an optical arrangement for transmitting short laser pulses in optical fibers, especially for insertion in a laser microscope, preferably for use in 2-photon laser microscopy using a short pulse laser source and at least one optical fiber, an optical device for changing the time of the laser pulses being provided between the laser light source and the optical fiber.
The insertion of short laser pulses in a laser microscope is preferably accomplished using an optical fiber, thereby simultaneously achieving high stability and flexibility. In the transmission of ultra-short light pulses, essentially two interfering effects have appeared in practice.
On the one hand, the light pulses are extended by fiber dispersion, typical values being in about 1 ps/m at 100 fs pulses. On the other hand, even at an average output of a few milliwatts, disturbances in the transmission of pulses occur due to the non-linear effects resulting from the high energy density in the fiber. This primarily involves self-phase modulation (SPM).
In known arrangements for the transmission of short pulses using optical fibers, one is already confronted in practice by undesired extension of pulses as a result of positive fiber dispersion, namely due to the side effect of a known corresponding negative dispersion in front of the optical fiber. Such a negative dispersion in front of the fiber can be realized, for example, by using prisms or gratings or grating systems. In the process, the pulse width at the end of the fiber is set to the original pulse width or to a longer pulse width. One may refer to German Patent 196 22 359 A1 as just one example.
However, if one sets the original pulse width using appropriate measures, the self-phase modulation effects limit the output of light transmission to a few milliwatts. In practice, this is not acceptable.
A method is known from Atherton B. W., Reed M. K., Proceedings of SPIE Vol. 3269, 1998, xe2x80x9cPrechirped fiber transport of 800 nm 100 fs pulsesxe2x80x9d, according to which the light pulse only past the fiber is minimized to the original pulse width using an additional optical element so that the fiber does not xe2x80x9cseexe2x80x9d short light pulses at any position. Consequently, there are no self-phase modulation effects, or only minimal ones.
The structural measures cited above have been used in the prior art for short-pulse transmission in optical fibers, but have a complex design and furthermore are expensive to adjust. In particular these arrangements are always wavelength-dependent. Due to both the complicated structure and the expensive adjustment, the known arrangements appear to be only marginally appropriate.
The object of the present invention is to configure and further develop an optical arrangement for the transmission of short laser pulses in optical fibers such that undesired fiber dispersion and non-linear effects as a result of high energy densities, especially self-phase modulation, are substantially prevented or compensated using simple means.
The optical arrangement according to the invention for transmission of short light pulses in optical fibers, meets the aforesaid objective by implementing an optical device between the laser light source and the optical fiber for changing the time of the laser pulses. The optical arrangement is characterized in that the optical device for changing the time of the laser pulses is configured as a segment consisting at least substantially of a transparent, dispersive medium in the beam path in front of the optical fiber and is used to extend the time of the laser pulses (pulse broadening).
According to the present invention, it is first recognized that an extension of the light pulses is required in order to prevent the aforementioned problems. Accordingly, a segment, made of glass for example, which is used to extend the time of the light pulses, is provided between the short-pulse laser light source and the input of the optical fiber. A segment of this typexe2x80x94in the beam path of the short-pulse laser light in front of the optical fiberxe2x80x94ensures that the material dispersion within the range of normal dispersionxe2x80x94under about 1500 nm wavelengthxe2x80x94causes the red portion of the light pulses to run faster than the blue portion. Accordingly, a xe2x80x9csmearingxe2x80x9d of the time of the laser pulse takes place. The degree of pulse dispersion therefore depends on the length of the segment and on the dispersion parameterxcex22 of the medium used.
The segment provided in the beam path in front of the optical fiber could be configured by a transparent container filled with a suitable liquid. The liquidxe2x80x94transparent and dispersivexe2x80x94in an advantageous way could be a silicone oil.
In principle, the segment could be designed as an optically homogenous isotropic medium. In concrete terms, the medium could be a solid body, for instance an isotropic crystal or plastic with corresponding properties.
Within the context of an especially beneficial exemplary embodiment, the segment or the medium is formed of glass, whereby in concrete terms it can be a glass body. This glass body can in turn be configured as a cylindrical glass block or as a cubic glass block, which in the simplest exemplary embodiment is configured as a monolithic block.
It is also conceivable that the optical device may comprise a plurality of modular glass blocks that can be combined into one overall block, both the monolithic single block and the glass blocks that can be combined into one block being interchangeable. Any definable segment can be made out of glass, the modular construction providing for alteration of, on the one hand, the dispersion parameter and, on the other hand, the length of the segment. In particular a combination of two wedge-shaped glass blocks in the sense of a sliding wedge pair, which enables a simple variation of the overall block length in a beneficial manner, is conceivable. The glass block or blocks in this context can be interchangeable.
In an especially beneficial manner, the optical device has a large dispersion parameter xcex22. A dispersion parameter this large offers the possibility of keeping the length of the segment as small as possible. Thus, the optical device could, for example, be configured so that an approximately 80 femtosecond light pulse can be expanded to about 280 femtoseconds by using a glass segment only 5 cm long. In this way the threshold for self-phase modulation in optical fibers is increased by an approximate factor of 3.5.
The glass selected for the medium must have suitable optical characteristics. Thus, in this context it can be a special type of glass. Namely, in a beneficial way, this can be the optical glass SF10 (heavy flint 10).
If retro-reflections of the laser light on the fiber end return back into the laser light source, under certain circumstances, this leads to disturbances in the laser operation. In this respect, the optical device can be formed in a beneficial way as a Faraday rotator or as a combination of a Faraday rotator with an additional optical device. The Faraday rotator permits effective use with respect to a wide adjustment band of the emission wavelength of the laser light source. The Faraday rotator is also capable of suitably rotating the polarization of the emitted laser light so that the polarization direction of the retro-reflection of the optical fiber situated downstream in the beam path is rotated back to the laser by the same amount. In this way the returning light can be suppressed by a linear polarizer arranged between the laser light source and the Faraday rotator. The polarizer is oriented parallel to the direction of polarization of the laser light emitted by the laser light source.
In order to minimize the pulse spread due to dispersion, which is added to the pulse spread due to the glass segment, a relatively short optical fiber should be used, for example an optical fiber with a length of 0.5 m. In this respect, it is possible to maintain pulse widths of scarcely under 1 ps at the end of a 0.5 m long optical fiber.
The arrangements known from the related art use so-called picosecond lasers specifically in order to keep both the self-phase modulation effects and the dispersion effects as small as possible. Accordingly, titanium-sapphire lasers mostly working in the picosecond range are used, which are more technically complex and therefore more expensive than titanium-sapphire lasers working in the femtosecond range. When seen from the functional principle of the titanium-sapphire laser, pulse widths in the range of 100 femtoseconds must be viewed as preferred pulse widths, making it possible to use more economical and compact femtosecond lasers. These lasers can be used according to the claimed concept in a trouble-free manner, the disadvantages known from the prior art being effectively prevented.